Species Description:The marsh periwinkle, Littorina irrorata,
is an abundant snail in the salt marshes of the western Atlantic.
The shell is elongate conic in shape, longer than it is wide
(Andrews 1994). Coloration of the shell is dull grayish white
with tiny dashes of reddish brown on the ridges of the spiral.
Eight to ten gradually increasing flat whorls comprise the shell,
with the body whorl measuring about half of the total height.
The aperture is oval with a sharp outer tip and regular grooves
on the inside edge.

Potentially Misidentified Species:Many of the species of littorinids common
to the western Atlantic are found in the IRL, including: the
mangrove periwinkle, Littorina
angulifera; slender periwinkle, L. angustior;
lineolate periwinkle, L. lineolata; white-spot periwinkle,
L. meleagris; and the zebra periwinkle, L. ziczac.
All of these species share a similar shell shape and an intertidal
distribution.

The mangrove periwinkle
attains a shell length of about 2.5 to 3.0 cm, and varies in
background color from bluish white, orange to dull yellow, reddish
brown to grayish brown (Andrews 1994). The shell is comprised
of 6 whorls, with the body whorl about half of the total height
of the snail. Darker dashes on the ribs of the shell are often
fused to form stripes on the body whorl. The early whorls around
the base bear regularly-spaced vertical white spots below the
channeled sutures.

The slender periwinkle
is relatively small, reaching a length of about 0.8 cm (Abbott
1974). The upper whorls of the shell are marked with 6-9 spiral
lines, the sides of the foot are mottled black and gray, and
the operculum is mostly round in shape.

The lineolate periwinkle
reaches a length of about 1.2 to 2.5 cm, has a gray background
color on the shell with oblique zigzag lines of dark brown,
and an apex of reddish brown (Andrews 1994). The shell is composed
of 6-8 gradually increasing whorls, with the body whorl spanning
more than half of the total length, and the suture between whorls
is well marked. The pear-shaped aperture has a sharp, thin outer
lip meeting the body whorl at an acute angle. Shells of males
are smaller and more strongly sutured then females.

The white-spot periwinkle
is also small like the slender periwinkle, measuring about 0.8
cm in length (Abbott 1974). The shell has a pointed spire with
a thin periostracum or organic covering. The aperture is reddish
brown and the exterior of the shell is brown with large, irregular
white spots, often arranged in spiral roles.

The zebra periwinkle
has a shell length of about 1.3 cm, and is whitish with dark
brown or black wavy stripes (Andrews 1994). The aperture is
small and oval, and the operculum is chitinous. This species
is often confused with L. lineolata, but has a lighter
colored shell with a narrower apical angle than the lineolate
periwinkle.

II. HABITAT AND DISTRIBUTION

Regional Occurrence & Habitat
Preference:The marsh periwinkle ranges from New York
to Texas (Abbott & Morris 1995, Bequaert 1943), and shares
a similar distribution with the salt marsh cordgrass, Spartina
alterniflora (Hamilton 1978). Individuals are found above
the water line on and around vegetation throughout salt marsh
areas (Hutchens & Walters 2006), often on dead, upright
leaves of Spartina alterniflora (Crist & Banta
1983). Occasionally, populations are found on jetty rocks and
seawalls (Gosner 1978).

IRL Distribution:The distribution of L. irrorata
is mainly limited to salt marsh habitats of the IRL, mostly
confined to the northern areas of the lagoon. In south Florida,
including the southern areas of the IRL, L. irrorata
is largely replaced by the mangrove periwinkle, L. angulifera
(Kaplan 1988).

III. LIFE HISTORY AND
POPULATION BIOLOGY
Age, Size and Lifespan:The maximum age of L. irrorata
is unknown, and the lifespan can vary with food availability
and environmental factors. The maximum reported shell length
for the marsh periwinkle is 3.2 cm (Kaplan 1988), but most individuals
are around 2.5 cm (Andrews 1994).

Abundance:The marsh periwinkle can reach high densities
in the salt marshes of the east coast of North America. In many
areas, populations range between 15 and 66 individuals m-2
(Hutchens & Walters 2006, Schindler et al. 1994).
However, when left unchecked be predators, they may approach
densities over 2,600 snails m-2 (Hutchens & Walters
2006).

Reproduction:Reproductive strategies are quite diverse
within the Littorina genus. Some species release egg
masses from with larvae hatch, others attach egg masses to hard
substrata, and some brood their young until giving birth to
larvae or juvenile snails (Ruppert & Barnes 1994).

IV. PHYSICAL TOLERANCES

Temperature:The large range and distribution of L.
irrorata throughout temperate to tropical latitudes suggests
the species has a wide thermal tolerance. Laboratory experiments
on the marsh periwinkle successfully exposed the snail to a
temperature range of 5 to 45 °C, although depressed respiration
rates suggested thermal stress at the extremities of this range
(Shirley et al. 1978). The ability of the marsh periwinkle
to attach to a surface via a mucous holdfast and withdraw completely
into its shell is likely a means of escaping extreme temperatures
and desiccation (Andrews 1994, Shirley et al. 1978).

Salinity: The salinity tolerances of L. irrorata
are poorly documented, but most populations are found in low
salinity waters (Andrews 1994).

Associated Species:No known obligate associations exist for
L. irrorata. However, marsh periwinkles are associated
with several organisms common to salt marshes and other intertidal
areas. For extensive lists of other species found in the habitats
in which L. irrorata occurs, please refer to the “Habitats
of the IRL” link at the left of this page.

VI. SPECIAL STATUS

Special Status:None

Ecological Importance:High densities of marsh periwinkles have
the potential to drastically reduce coverage of their main food
source, the cordgrass, Spartina alterniflora. As a
top predator of L. irrorata, the Atlantic mud crab,
Panopeus herbstii, helps to keep snail populations
in check that could otherwise decimate salt marsh vegetation
(Silliman et al. 2004).